International Application PCT/CH90/00283 (U.S. Pat. Ser. No. 07/741,531) now U. S. Pat. No. 5429339 discloses a warp-thread drawing-in machine in which the thread layer is clamped on a frame, and the individual warp threads are separated one after the other, offered to a drawing-in gripper and drawn by the latter into the harness. After being received by the drawing-in gripper, the vertically clamped warp thread, in loop form, passes a vertical thread channel at the beginning of which a thread monitor is arranged. The thread monitor checks during every draw-in whether a warp thread has actually been drawn in. It is made up of a flexible pivoted lever which can be displaced by the passing thread, and the displacement of the pivoted lever is monitored by an inductive sensor.
This thread monitor has proved successful in practice, but it still does not represent an optimum solution to the thread passage detection task. The pivoted lever is subject to a certain wear and stress. It is also not out of the question for the inductive sensor to function unreliably during heavy accumulation of dust. Since as high a reliability as possible is required of this thread monitor, its operation requires a certain amount of maintenance, which is always undesirable.